Conventional cellular area systems are deployed with mobile terminals that have the capability to mitigate interference. Mitigating interference has the effect of improving the signal to interference plus noise ratio (SINR) measured at the output of the receiver, resulting in better performance. This capability can be used, for example, in detecting transmitted data information or in computing some measure of channel quality information (CQI) that is used for link adaptation and user scheduling.
Canceling interference typically requires the estimation of interferer parameters. The process of interference parameter estimation is more easily accomplished when the interferer does not vary significantly in time and/or frequency due to fading, dispersion or intermittent transmission of the interferer. Since the estimation process typically is performed in the presence of random (i.e. thermal) noise, a sufficient number of estimation samples are required to minimize the effects of the noise. Further, estimation degrades when those samples are used where the interference varies abruptly.
In OFDM (Orthogonal Frequency Division Multiplexing) systems, such as LTE (Long Term Evolution), data symbols are transmitted according to a time-frequency grid, where a grid element is referred to as a resource element (RE). Resource elements are grouped together into larger groups called resource blocks (RBs). Certain resource elements are allocated within a resource block for performing channel estimation. These are commonly referred to as reference REs or pilots. During the detection process, channel estimates are obtained at the location of each data RE. However, since knowledge of interferer pilots is unknown, interferer parameters, in the form of the interferer covariance, are estimated once per resource block and used for interference-rejection combining (IRC).
During detection of the nth data resource element, the maximum-likelihood (ML) combining weight vector, wmL,n, using the IRC approach can be formulated as given by:wML,n=Re−1GnPn,  (1)where Gn is the channel estimate at the nth data RE, Pn is the precoding in effect for that data RE, and Re is the estimated impairment covariance parameter matrix. The impairment covariance, Re, is estimated as given by:
                                          R            e                    =                                    1              K                        ⁢                                          ∑                                  k                  =                  1                                K                            ⁢                                                          ⁢                              e                k                                                    ,                            (        2        )            where ek is the residual error between the received data for the kth pilot in a resource block and the estimated received data for that pilot position. In this case there are assumed to be K pilots within a resource block. The computed weights are applied to the received data yn for this data RE to form the detection statistic dn as follows:dnwML,nHYn.  (3)Alternate forms of computing the detection statistic are to compute the MMSE combining weight estimate using the data covariance.
It is preferred to use as many pilots as possible (i.e. as large a K in equation (2) above as possible) to compute the interferer covariance, provided that those pilot positions share the same interference environment. In practice, this is not feasible for several reasons. First, the interference is temporally intermittent so incorporating pilots from multiple subframes to perform the estimation is not reliable. Even if interference is present in a previous subframe, it may arise from a different interferer. Additionally, even if the same interferer is present, rapid channel fading may cause the interference environment to change. Second, implementation constraints may limit the number of pilots within a subframe that can be used for impairment covariance estimation. For example, to reduce latency, only pilots from the first of two slots within a subframe may be available to perform this estimation, effectively cutting in half the number of useful pilots. Further, pilots located within the first three OFDM symbols of a subframe (assuming synchronous desired and interferer transmissions) may overlap with the interferer's control channel. Since the transmission formats on the control and data channels may be different, those pilots that overlap the control channel will see a different interference environment (and may not help in reliably estimating the interferer covariance on the data REs). Third, pilots from adjacent resource blocks observe a different interference environment because dispersion varies the propagation channel across frequency. Fourth, the precoding applied to interferer transmission can vary across resource blocks, again changing the interference environment.